The phrase "a commonplace but" is correct and usable in written English.
It can be used to introduce a statement that is widely accepted or known, but then contrasts it with a different or unexpected point.
Example: "It is a commonplace but often overlooked fact that exercise is essential for good health."
Alternatives: "a well-known but" or "a familiar but".
It's still a commonplace, but it's changing shape.
Coming into contact with pornography is a commonplace but often unwelcome experience for children and young people.
The plot point may be a commonplace, but Johnson manages to lend it a spontaneous, naïve resonance.
That our technologies alter our posture and our experience of our own bodies is a commonplace, but it's fascinating to envision the differences splayed across entire societies.
The existence of multiple forms of intelligence has become a commonplace, but however much elite universities like to sprinkle their incoming classes with a few actors or violinists, they select for and develop one form of intelligence: the analytic.
It is, by now, a commonplace, but one that bears repeating: in 1956, Lewis had the idea for a closed-circuit TV camera to be mounted beside the movie camera so that, as a performer, he (and not just the director) could see the effect he was achieving on-screen, and he submitted a patent application for such a system.
It is now a commonplace -- but for all that still unnerving -- that it was very often not merely the stupid but the highly intelligent who gave their support to the Hitlers and the Stalins of the last century.
The bedtrick may not really be a commonplace occurrence, but it is a commonplace preoccupation.
It's become a bit of a commonplace since, but I believe we were among the first to treat people not just as paying customers, but as part of the performance".
To be sure, he had no musical aptitude and a commonplace voice, but he was forever whistling the latest hit tunes, as though this might somehow lead him closer to his ambition.
Tarantino, as usual, is profligate in placing that word in his characters' mouths, but — at least in this film — it's used not gratuitously as a commonplace epithet but hatefully by hateful characters who intend to demean and dehumanize the Major.
